1. Field of the Invention
This invention relates to steam heat exchangers for wood pulp dryers.
2. Description of the Related Art
Pulp dryers are conventionally used to dry wood pulp in sheet form prior to shipment. Heat is applied to the pulp by heating air with heat exchangers, normally heated with steam, and blowing the air against the web of pulp. The type of heat exchanger used on many dryers built have a frame with a plurality of spaced-apart, vertical copper tubes extending between opposite ends thereof. The tubes extend tightly through fins, usually of aluminum, which are perpendicular to the tubes. The tops of the tubes are connected to an inlet header which is typically perpendicular to the tubes. The bottoms of the tubes are connected to an outlet header. Steam is conventionally fed into the heat exchangers via T fittings on vertical steam pipes.
However, the life span of such prior art heat exchangers has been less than desirable. In some cases the life expectancy has been approximately 8 years, whereas a life span of 16-20 years is expected by many in the pulp industry. Pulp dryers have many heat exchangers and it is expensive to replace them, both in terms of the cost of the new heat exchangers, the labor required to replace them, and the down time of the pulp dryer needed to carry out this maintenance operation.
The failure of some prior art heat exchangers is believed to be due to a combination of problems. In addition, the arrangement of the inlet header appears to cause an uneven distribution of steam in the various tubes of each heat exchanger. The tubes carrying higher steam flow wear faster. In some mills steam becomes contaminated with chemicals and by-products of the pulp conversion process from time to time, the most common being known as black liquor. This material coats and ultimately corrodes the tubes.
In my earlier Canadian Patent No. 2,040,827 and U.S. Pat. No. 5,782,293 I disclosed the concept of a steam coil with an inlet header having inner and outer conduits. The inner conduit has openings in the top which allow steam to pass into the space between the inner conduit and outer conduit which is connected to the tubes of the steam coil. Impurities are diverted along the inner conduit to a diverter conduit to reduce possible contamination of the tubes with black liquor or other contaminants. The openings in the top of the inner conduit are louvered or tear-drop shaped.
However, the life span of heat exchangers or steam coils has still been less than desirable. I have ascertained that this is due to a number of factors. On factor is that distribution of steam is less than optimal among the different tubes of each heat exchanger. This leads to accumulation of condensation and premature wear of the tubes.
This factor is particularly notable in heat exchangers having three rows of tubes. The majority of heat exchange occurs with the first row of tubes which is exposed first to the air. Accordingly, most of the condensation occurs in the first row. A relatively small amount of heat exchange occurs in the third row of tubes, for example about 9% of the heat exchange in some examples. Little condensation therefore occurs in the third row. Steam is typically fed from the inlet header at the tops of the tubes. This tends to go straight down the third row of tubes and then back up through the first and second rows of tubes. This holds up condensate in the first and second rows due to the reverse flow of steam coming from the bottom instead of the top. The condensate held up in the first and second rows causes premature erosion and corrosion of the tubes and therefore premature failure of the steam coils.
Another problem is caused by mechanical failure of inlet headers of the type having two concentric tubes. This failure often occurs adjacent the end of the header which is opposite the inlet end. This occurs because of differential expansion and contraction between the inner and outer tubes and the structure connecting them.
Accordingly it is an object of the invention to provide an improved heat exchanger for pulp dryers which provides better distribution of steam from the inlet header to the tubes.
It is also an object of the invention to provide an improved heat exchanger for pulp dryers having two or more rows of tubes, such that short circuiting of steam is inhibited up through the rows of tubes first exposed to the flow of air. This prevents uncondensed steam from the last row of tubes from entering the bottom of the first row of tubes.
It is a further object of the invention to provide an improved heat exchanger for pulp dryers, of the type having concentric tubes, whereby mechanical failures between the concentric tubes is reduced.
In accordance with these objects there is provided, according to one aspect of the invention, a steam heat exchanger for a wood pulp dryer. The heat exchanger has a first end, a second end which is opposite the first end and includes a plurality of tubes. Each tube has an inlet end and an outlet end. The tubes extend between the ends of the heat exchanger in parallel, spaced-apart relationship to each other. There is an outlet header near the second end of the heat exchanger. The outlet ends of the tubes are connected to the outlet header. There is a steam inlet header near the first end of the heat exchanger. The inlet header has an inner conduit with an outer surface, a central axis, a connector at a first end thereof for connecting the inner conduit to a source of steam and a plurality of openings spaced-apart therein for discharging steam from the inner conduit. An outer conduit extends about the inner conduit. The inner ends of the tubes are connected to the outer conduit. The openings in the inner conduit each have a first end and a second end. The first end of each opening is closer to the first end of the conduit than the second end of each opening. The first end of each opening is firther from the central axis of the inner conduit than the second end thereof.
According to another aspect of the invention there is provided a steam heat exchanger for a wood pulp dryer. The heat exchanger has a first end, a second end which is opposite the first end and includes a plurality of tubes. Each tube has inlet end and an outlet end. The tubes extend between the ends of the heat exchanger in parallel, spaced-apart relationship to each other. There is an outlet header near the second end of the heat exchanger. The outlet ends of the tubes are connected to the outlet header. There is a steam inlet header near the first end of the heat exchanger. The inlet header has an inner conduit with an outer surface, a central axis, a first end, a second end opposite the first end, a connector at the first end thereof for connecting the inner conduit to a source of steam and a plurality of openings spaced-apart therein for discharging steam from the inner conduit. An outer conduit extends about the inner conduit. The inlet ends of the tubes are connected to the outer conduit. The outer conduit has a closed end adjacent the second end of the inner conduit. There is a mount for the inner conduit which is fixedly connected to the closed end of the outer conduit and which slidably engages the second end of the inner conduit.
There is provided, according to a further aspect of the invention, a steam heat exchanger for a wood pulp dryer having a first end, a second end which is opposite the first end, a first side onto which air is directed and a second side which is opposite to the first side. There is a plurality of tubes, each having an inlet end and an outlet end. The tubes extend between the ends of the heat exchanger in parallel, spaced-apart relationship to each other. The tubes are arranged in a first row and a second row. The first row is adjacent to the first side of the heat exchanger and the second row is further from the first side than the first row. There is an outlet header near the second end of the heat exchanger. The outlet ends of the tubes are connected to the outlet header. There is a steam inlet header near the first end of the heat exchanger. The inlet ends of the tubes are connected to the inlet header. A flow restrictor is operatively located between the inlet header and the outlet ends of tubes of the second row. A flow of steam through the second row of tubes is reduced compared to a flow of steam through the first row of tubes. Preferably the flow restrictors are adjacent the inlet ends of the tubes. Preferably there is a third row of tubes between the first and second rows.